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A key report into allegations of Russian doping at the 2014 Sochi Winter Games may have been undermined by a leaked letter to the International Olympic Committee.

The McLaren report, headed by Canadian law professor Richard McLaren, is due to be released early tomorrow. Although it focusses on violations in Sochi, it has a potentially far wider impact with calls growing for Russia to be thrown out of the Rio Olympics next month.

The letter, penned by United States Anti Doping Agency chief executive Travis Tygart and his Canadian counterpart Paul Melia, was also circulated to Wada's Athlete Committee.

It suggests at least 10 national anti-doping organisations - reported to include the United States, Germany, Japan, New Zealand and Canada - and more than 20 athlete groups representing Olympians from around the world have grouped together to request a complete ban on Russia in anticipation ahead of the McLaren report.

However a World Anti Doping board member, Nenad Lalovic, has criticised the leaking of a letter, which was due to be presented to the IOC after the report comes out.

Lalovic, who heads United World Wrestling and is a member of Wada's foundation board, argues it will hurt the prospects of a Russian ban.

''Unfortunately this report is harmed by all the events before the report is public," Lalovic said. ''Obviously the report focuses on the situation in Russia but we cannot generalise. Of course it has been harmed now.

''Nobody will now take it as seriously as it should be because it seems that a lot was known in advance."

The report, commissioned by Wada, is expected to confirm the allegations made by Grigory Rodchenkov, the former director of Moscow's Wada-accredited anti-doping laboratory, that he effectively sabotaged the Sochi Winter Olympics, doping dozens of athletes, including at least 15 medallists, in the build-up to the event.

Rodchenkov claimed he was working as part of a government-orchestrated plot, with Russian security services helping him open and reseal supposedly tamper-proof doping sample containers before removing samples from his laboratory through a concealed hole in a wall at night. Russia denies the allegations.

''If we have drafts of letters being circulated asking to ban all Russian athletes then that is strange," Lalovic, who is also an IOC member said.

''USADA should be focused on the health of American athletes and those competing in the United States. Now it seems that USADA and the Canadians took over responsibility of Wada. Nobody entitled them to do that."

Part of the leaked letter reads: "We write on behalf of a community of clean athletes and anti-doping organisations with faith that the IOC can lead the way forward by upholding the principles of Olympics."

Further criticism of the leak has come from swimming's world body Fina.

''Fina is concerned by the premature calls from US and Canadian Anti-doping authorities to ban Russia from Rio," it said.

''Fina is also concerned that there has been a drive behind the scenes...to get a global coalition .... to support the call for the total ban on Russia.

''All of this is based on the findings of the McLaren Report, which is meant to have been independent and confidential.

''Such breaches of confidentiality and the perception of a breach of independence of the report undermine its credibility."

The IOC is facing potentially one of its biggest decisions.

Russia's track and field team has already been banned by the international athletics body, the IAAF.

It appears the IOC is preparing to take the hardest of lines against Russian athletes. Russia is traditionally one of the biggest Games medal gatherers but there's a feeling the IOC has drawn a line.

If McLaren's report details widespread doping out of Sochi, and beyond, and with the track and field team rubbed out of Rio, this is poised to be among the blackest of days for the Olympic movement.